Matrix-type fuel cell

ABSTRACT

A compact electrochemical cell is described comprising an anode, a cathode, a matrix containing an ion-conductive electrolyte between the anode and cathode, and porous metal plates containing porous pins positioned adjacent each of said anode and cathode in order that the pins of the plates are in contact with said anode and cathode over the limited surface area of the pins. The electrolyte volume of the cell is controlled by electrolyte movement through the pins of the porous plate, thereby stabilizing the electrochemical performance of the cell.

[45] Dec. 18, 1973 United States Patent [191 Bushnell et al.

3,370,984 2/1968 Platner...............,.............. 136/86 C3,418,168 12/1968 MATRIX-TYPE FUEL CELL Wentworth 136/86 R y e k a H .mt mm .m m BB 6 am mm E WW am .mo nu PA [73] Assignee: United AircraftCorporation, East Hartford, Conn.

Mar. 16, 1971 ABSTRACT A compact electrochemical cell is describedcompris- [22] Filed:

l a nod a c thod a matrix cont 'ni an Appl. No.: 124,862 n a e a 6 a1 ngion conductive electrolyte between the anode and cathode, and porousmetal plates containing porous pins positioned adjacent each of saidanode and catho de in order that the pins of the plates are in contactwith said anode and cathode over the limited surface area 26 08 WW N23 61 m. 1 0

. c n r n a n e H S L -m W d S Uhh H M- 555 of the pins. The electrolytevolume of the cell is con- R f r Ci trolled by electrolyte movementthrough the pins of UNITED STATES PATENTS the porous plate, therebystabilizing the electrochemical performance of the cell.

3,442,712 Roberts, 3,507,702 4/1970 Sanderson.... 10 Claims, 2 DrawingFigures /Q El L FA .1 P 7. 15m $1 .5 16 t azk rfipa fxtfl j ki /11% yifwi s a .9 v 6. .ll: I M ib Eb Lifiiiliiim will i: 1:113:11? \fiQw W O 4S m 3 Q 0 4 J w MATRIX-TYPE FUEL CELL FIELD OF INVENTION AND BACKGROUNDThis invention relates to electrochemical cells and, more particularly,to an improvement in an electrochemical cell utilizing an electrolytecontained'in, or trapped in a matrix between the electrodes of a cellwhereby the volume of the electrolyte is controlled, stabilizing cellperformance. For convenience, hereinafter the invention will bedescribed with reference to a fuel cell for the direct generation ofelectricity utilizing two non-consumable electrodes. As will beapparent, however, similar considerations governing the use of theinvention in such cells will apply to other electrochemical devices suchas electroylzers enabling its use in such devices.

A fuel cell, as the term is employed herein, designates anelectrochemical cell for the direct production of electrical energy froma fuel and oxidant. With such cells, it is not necessary to go throughthe usual conversion of chemical energy to heat energy to mechanicalenergy to electrical energy as is common with heat engines. Such cellsin their most simplified design comprise a housing, an oxidizingelectrode, a fuel electrode, and an electrolyte. In operation, it isnecessary that the fuel and oxidant contact a surface of theirrespective electrode where a process of adsorption and deadsorptionoccurs leaving the electrodes electrically charged, with the secondsurface of .the electrodes being in contact with the electrolyte.Depending upon the nature of the electrolyte, ions are transferredthrough the electrolyte from the anode to the cathode, or from thecathode to the anode. Electrical current is withdrawn from the cell andpassed through a suitable load where work is accomplished.

Although the electrolyte can be a solid, a molten paste, a free-flowingliquid, or a liquid trapped in a matrix, as a result of designconsiderations including compactness and the desire to have a limitednumber-of controls and ancillary equipment, cells utilizing a liquidelectrolyte trapped in a hydrophilic matrix-are preferred for manyapplications. A problem of such cells, however, is the change inelectrolyte volume in the matrix as a result of water being formed bythe interaction of the fuel and oxidant and/or as a result ofelectrolyte loss through excessive heating of the cell or use of dryreactants during operation of the cell. In instances where theelectrolyte is increased, the excess electrolyte is carried by capillaryaction into the electrodes of the cell with resultant flooding of theelectrodes. In instances where the volume of electrolyte is decreased,-dry-out will occur at the electrolyte matrix-electrode interface. Suchflooding and/or dry-out adversely affects the electrochemicalperformance of the cell.

' In the prior art, to compensate for the change in electrolyte volumein a trapped electrolyte cell, the use of electrodes comprising asintered metal normally 30 to 50 mils thick has been suggested. Thethick metal sinter is to compensate for the increase in volume of theelectrolyte during operation of the cells. As readily apparent, however,the aforesaid solution cannot compensate for dry-out; and, furthermore,the thick electrodes with the changing electrolyte interface caused highand fluctuating IR loss across the cell varying the electricalperformance of the cell. Obviously, the use of thick electrodes resultedin relatively thick or bulky cells.

OBJECTS OF THE INVENTION AND GENERAL DESCRIPTION Accordingly, a primaryobject of the present invention is to provide a cell design whichseparates the volume tolerance function from the electrochemicalfunction of the cell.

Another object of the present invention is to provide a matrix-type fuelcell which permits convenient removal of excess liquid, preventingflooding of the electrodes.

Another object of this invention is to provide a matrix-type fuel cellwhich permits convenient replenishing of electrolyte, preventingmatrix/electrode dry-out.

Another object of this invention is to provide a matrix-type fuel cellhaving an electrolyte reservoir which will automatically control theelectrolyte volume in the cell matrix.

Another object of this invention is to provide a matrix-type fuel cellhaving improved cell spacing.

Another object of this invention is to provide a matrix-type fuel cellhaving a low IR loss.

These and other objects of the invention will be more readily apparentfrom the following detailed description, with particular emphasis beingplaced on the embodiment illustrated in the drawing.

In accordance with the present invention, a matrixtype fuel cell isconstructed which incorporates a porous plate having a series orplurality of porous pins or ridges behind either one of the anode orcathode, or behind both the anode and cathode. Theporous pins or ridgesare in contact with the electrode or electrodes of the cell. Theelectrolyte from the electrolyte matrix floods these pins and is free tomove back and forth between the porous plate and cell matrix through theelectrode as the electrolyte volume changes. Accordingly, theelectrolyte volume of the electrochemical cell is always constant,avoiding fluctuations in the cell performance as a result of electrolytevolume change. More specifically, as the electrolyte within the cellincreases as a result of water formation during the cell reaction, theamount of electrolyte in the porous plate will increase; or if theelectrolyte decreases as a result of excessive heat or reactant flow,electrolyte will flow from the porous plate to the matrix, decreasingthe electrolyte in the porous plate. However, the electrolyte within thecell matrix will remain constant. Effectively, therefore, the porousback-up plate will function as a reservoir feeding'electrolyte to theelectrolyte matrix on demand, or withdrawing or removing electrolytefrom the matrix as it is formed.

In. operation of the cell, the reactant gas will be passed to theelectrodes'between the porous back-up plate and the electrode. Thegaseous reactant will be interrupted as a result of the pins and/orridges on the porous plate, improving reactant circulation and reactantcontact with the electrode. The porous plate will also function as thecurrent collector for the electrode. If desired, in the event theoperating conditions of the cell are such that electrolyte build-upbeyond the ca-' pacity of the porous plate is likely,-a cooling platecan be placed behind the porous plate and a cooling gas circulatedbetween the cooling plate and porous plate to remove excess water as itis formed. On the other hand, in the event operating conditions of thecell are such that the electrolyte will need to be replenished, moisturecan be added to the porous plate and, thus to the cell matrix, byfeeding electrolyte or water to the back of the porous plate either as avapor or as a liquid. Effectively, therefore, in accordance with thepresent invention, the electrolyte volume of the cell is maintainedconstant, assuring stability of cell performance.

THE DRAWING AND SPECIFIC EMBODIMENT In order to more specificallydemonstrate the present invention, reference is made to the accompanyingdrawing wherein FIG. 1 is a transverse sectional view through a singlefuel cell constructed in accordance with the present invention; and

FIG. 2 is a graph illustrating electrolyte volume tolerances of amatrix-type fuel cell.

Referring to FIG. 1 of the drawing, the fuel cell 10 comprises anode 5and cathode 7 separated by an electrolyte matrix 6. In the embodimentshown, electrodes 5 and 7 are lightweight screen electrodes comprising aconductive nickel screen embedded in a uniform admixture of catalyticmetal, in this instance platinum, and polytetrafluoroethylene particles.Th ratio of platinum to polytetrafluoroethylene on a volume basis is3:7, with the platinum loading of the electrode being l5mg/cm Theelectrodes are approximately 10 mils in thickness. The electrolytematrix is pressed asbestos and is approximately 25 mils thick. A porousplate having a plurality of porous pins 22 is adjacent to and incommunication with each of the anode and cathode through pins 22. In thepreferred embodiment shown, the plate is porous nickel having a totalporosity of about 80 percent. As apparent from the drawing, each of theporous plates is adjacent to a cooling plate 30. Cooling plates 30 areseparated from pressure plates 40 by insulation 35 and the entire cellassembly held together with threaded tie rods 38.

In operation, electrolyte matrix 6 is saturated with a 30 percentaqueous potassium hydroxide electrolyte through air inlet plug, notshown. Sufficient electrolyte is added in order that the electrolytewill pass into porous pins 22 and partially into porous plate 20. Apossible electrolyte interface is shown by dotted line 24 in plate 20behind anode 5. A reactant gas, in this instance hydrogen, is fed toanode 5 through gas inlet 5a, with excess gas being removed throughoutlet 5b. An oxidant, in this instance air, is fed'to cathode 7 throughinlet 7a, with excess air and impurities being vented through exit 7b.Depending upon the current characteristics and operating conditions ofthe cell, it may be desirable to cool the cell by passing a cooling gas,i.e., air, or a cooling liquid such as ethylene glycol, propyleneglycol, or glycerine between cooling plate 30 and end plate 40 through achannel, not shown.

Although in the embodiment shown in the drawing porous plates andcooling plates are shown behind each of the electrodes, it can bedesirable in order to conserve space to only have the porous nickelplate and cooling plate behind one of the anode or cathode with theelectrolyte volume in the matrix being controlled through this singleunit. As will be readily apparent, again depending upon operatingconditions, i.e., where the current drain is relatively low and theoperating temperatures of the cell are constant, it may not be necessaryto utilize a cooling plate at all. This, as will be apparent, willsubstantially save on the total weight of the cell and, further, willprovide a more compact cell.

The cell when operated at a constant current drain will provide asubstantially constant cell output. There is little fluctuation in thecurrent characteristics of the cell since the entire volume tolerancefunction is separated from the electrochemical function because of theuse of the porous pin plates. This is shown graphically in FIG. 2 of thedrawing. From the graph it is seen that the electrochemical output ofthe presently disclosed cell is substantially identical to the outputwhich is theoretically obtainable with a matrix-type cell. Note lines 1and 3. In contradistinction, without the porous pin plate, the currentcharacteristics of a matrix cell are changed substantially at both lowand high electrolyte volumes as a result of the varying electrolyteinterface in the electrodes and the varying effective electrochemicalarea of the cell electrodes. Accordingly, the advantages ofthe presentsystem are readily apparent.

Although the present invention has been described with reference tolightweight electrodes comprising a metal support screen embedded in acatalytic mixture of metal and hydrophobic plastic binder, otherelectrodes can be employed including non-porous-palladium/silver alloystructures as described in U.S. Pat. No. 3,092,517. Furthermore, theso-called Bacon-type electrode as defined in U.S. Pat. No. 2,716,670 canalso be employed. Although it is indicated that the electrolyte matrixis made of asbestos, other hydrophilic matrices including ceramicmaterials and polymeric materials cn be utilized. In addition to nickel,the porous back-up plate made by any conventional technique can be anymaterial which is hydrophilic, i.e., will collect water as a result ofcapillary action, and includes porous copper, tantalum, iron, and thelike. As a result of availability and over-all characteristics, nickelis preferred. The porosity of the plate can vary as long as it issufficiently porous to adsorb water through capillary action, butpreferably the plate will have a porosity of from about 35 to percent.The operating temperature of the cell can vary as long as it isnot'above the critical temperature of the electrodes and/or electrolytematrix being employed. Preferably, the operating temperature ofmatrix-type cells of the type described herein will range from about 20to C. In addition to the potassium hydroxide electrolyte disclosedhereinbefore, other commonly employed aqueous electrolytes exemplifiedby aqueous solution of the alkali hydroxides, alkaline earth hydroxides,and carbonates, as well as strong acid electrolytes such as hydrochloricacid, sulphuric acid, and phosphoric acid can be employed. Commonlyemployed reactants, in addition to hydrogen and oxygen, can be utilizedin the cells of the present invention. As will be apparent, the conceptof the present invention can be employed in any of the prior art cellswhere electrolyte volume control within a matrix-type electrolyte isessential.

Furthermore, although the present invention is described and illustratedin the drawing with reference to a single cell, it should be apparentthat in preferred constructions a plurality of cells will be stackedtogether; and where cooling is not necessary, the porous metal platescan have the porous pins or ridges on both sides and serve electrodes ofadjacent cells. This will increase the compactness of a battery ofcells. Alternatively, if cooling is necessary, a single cooling chambercan be positioned between two porous plates, with the cooling chamberservicing the two porous plates which are in contact with electrodes ofopposite cells. As will be apparent to those skilled in the art, variousmodifications can be made in the over-all design to meet operatingconditions. For example, .a stack of cells employing the concept of thisinvention .can be utilized with a humidity exchange/scrubber unit of thetype de- 1. A fuel cell having a pair of opposed electrodes, an

electrolyte matrix positioned between said pair of electrodes, aself-sustaining porous plate having a plurality of porous projectionspositioned behind at least one of said pair of electrodes and in contactwith said one electrode over the entire surface area of saidprojections; said matrix and said porous plate containing an aqueouselectrolyte and being in electrolyte communication with each otherthrough said electrode at the area at which said plurality ofprojections contact said electrode, said electrolyte filling less thanthe entire volume of said porous plate, whereby the electrolyte volumeof said matrix is maintained constant.

.2. The fuel cell of claim 1 including a back-up plate behind saidporous plate, said back-up plate and porous plate forming a gas coolantchamber therebetween, and reactant-fed means for feeding reactant toboth sides of said porous plate.

3. The fuel cell of claim 1 having a porous plate behind each of saidpair of electrodes.

4. The fuel cell of claim 3 wherein said porous plate is porous nickelhaving a porosity of from about 35 to 80 percent.

5. The fuel-cell of claim 4 wherein said projections of said porousplate are pins.

6. The fuel cell of claim 5 wherein said pair of electrodes arelightweight screen electrodes comprising a support screen in contactwith a catalyst mix of electrocatalyst and hydrophobic polymer binder.

7. The fuel cell of claim 6 wherein said binder is polytetrafluoroethylene.

' cation with each other through the electrode surface area at whichsaid plurality of projections contact said electrode, said electrolytecompletely filling said matrix and filling less than the entire volumeof said porous plate whereby the electrolyte volume of said matrix ismaintained constant, a back-up plate positioned behind said porous platewhich together with said porous plate forms a passage between saidporous plate and back-up plate, and means for feeding a gaseous reactantsimultaneously between said porous plate and said one electrode andbetween said back-up plate and said porous plate.

10. The fuel cell of claim 9 wherein porous plates are positioned behindeach of said pair of electrodes and said plates are electricallyconnected to an external cirelectrodes.

2. The fuel cell of claim 1 including a back-up plate behind said porous plate, said back-up plate and porous plate forming a gas coolant chamber therebetween, and reactant-fed means for feeding reactant to both sides of said porous plate.
 3. The fuel cell of claim 1 having a porous plate behind each of said pair of electrodes.
 4. The fuel cell of claim 3 wherein said porous plate is porous nickel having a porosity of from about 35 to 80 percent.
 5. The fuel cell of claim 4 wherein said projections of said porous plate are pins.
 6. The fuel cell of claim 5 wherein said pair of electrodes are lightweight screen electrodes comprising a support screen in contact with a catalyst mix of electrocatalyst and hydrophobic polymer binder.
 7. The fuel cell of claim 6 wherein said binder is polytetrafluoroethylene.
 8. The fuel cell of claim 7 wherein the electrolyte is an aqueous alkali hydroxide.
 9. A fuel cell having a pair of opposed electrodes, an electrolyte matrix positioned between said pair of electrodes, a self-sustaining porous plate having a plurality of porous projections positioned behind at least one of said pair of electrodes and in contact with said one electrode over the entire surface area of said projections; said matrix and said porous plate containing an aqueous electrolyte and being in electrolyte communication with each other through the electrode surface area at which said plurality of projections contact said electrode, said electrolyte completely filling said matrix and filling less than the entire volume of said porous plate whereby the electrolyte volume of said matrix is maintained constant, a back-up plate positioned behind said porous plate which together with said porous plate forms a passage between said porous plate and back-up plate, and means for feeding a gaseous reactant simultaneously between said porous plate and said one electrode and between said back-up plate and said porous plate.
 10. The fuel cell of claim 9 wherein porous plates are positioned behind each of said pair of electrodes and said plates are electrically connected to an external circuit for withdrawing electrical energy from said cell electrodes. 